Acknowledge `zxx` in BCP47 section

[marrus-sh]

Per https://www.w3.org/International/questions/qa-no-language#nonlinguistic, zxx is the correct language tag for strings known not to contain linguistic content.

This makes the following paragraph incorrect:

Note that none of the examples in this section contained an example for an ISBN term, which needs a base direction but no language tag. One might consider to use, e.g., und-x-d-ltr (where und is the BCP47 tag for an “undefined” language), but that may be at odd with the surrounding context and, therefore, is semantically not clean. In other words, it is not clear how the direction of an ISBN string could be defined in any of those schemes.

The correct tag would be zxx-x-d-ltr.

[iherman]

Thank you, @marrus-sh, this is is indeed correct.

However, that document is not evolved anymore; it was a discussion document that ended its course. The JSON-LD Working Group has now taken over the issue, see the i18n namespace in that document.

I will actually put this comment into the readme file of this repository and possibly archive it to avoid any misunderstanding.

@pchampin